RFC 8086

GRE-in-UDP Encapsulation

Internet Engineering Task Force (IETF) L. Yong, Ed.
Request for Comments: 8086 Huawei Technologies
Category: Standards Track E. Crabbe
ISSN: 2070-1721 Oracle
X. Xu
Huawei Technologies
T. Herbert
Facebook
March 2017 GRE-in-UDP Encapsulation
Abstract
This document specifies a method of encapsulating network protocol
packets within GRE and UDP headers. This GRE-in-UDP encapsulation
allows the UDP source port field to be used as an entropy field.
This may be used for load-balancing of GRE traffic in transit
networks using existing Equal-Cost Multipath (ECMP) mechanisms.
There are two applicability scenarios for GRE-in-UDP with different
requirements: (1) general Internet and (2) a traffic-managed
controlled environment. The controlled environment has less
restrictive requirements than the general Internet.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8086.

Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

1. Introduction
This document specifies a generic GRE-in-UDP encapsulation for
tunneling network protocol packets across an IP network based on
Generic Routing Encapsulation (GRE) [RFC2784] [RFC7676] and User
Datagram Protocol (UDP) [RFC768] headers. The GRE header indicates
the payload protocol type via an EtherType [RFC7042] in the protocol
type field, and the source port field in the UDP header may be used
to provide additional entropy.
A GRE-in-UDP tunnel offers the possibility of better performance for
load-balancing GRE traffic in transit networks using existing Equal-
Cost Multipath (ECMP) mechanisms that use a hash of the five-tuple of
source IP address, destination IP address, UDP/TCP source port,
UDP/TCP destination port, and protocol number. While such hashing
distributes UDP and TCP [RFC793] traffic between a common pair of IP
addresses across paths, it uses a single path for corresponding GRE
traffic because only the two IP addresses and the Protocol or Next
Header field participate in the ECMP hash. Encapsulating GRE in UDP
enables use of the UDP source port to provide entropy to ECMP
hashing.
In addition, GRE-in-UDP enables extending use of GRE across networks
that otherwise disallow it; for example, GRE-in-UDP may be used to
bridge two islands where GRE is not supported natively across the
middleboxes.
GRE-in-UDP encapsulation may be used to encapsulate already tunneled
traffic, i.e., tunnel-in-tunnel traffic. In this case, GRE-in-UDP
tunnels treat the endpoints of the outer tunnel as the end hosts; the
presence of an inner tunnel does not change the outer tunnel's
handling of network traffic.
A GRE-in-UDP tunnel is capable of carrying arbitrary traffic and
behaves as a UDP application on an IP network. However, a GRE-in-UDP
tunnel carrying certain types of traffic does not satisfy the
requirements for UDP applications on the Internet [RFC8085].
GRE-in-UDP tunnels that do not satisfy these requirements MUST NOT be
deployed to carry such traffic over the Internet. For this reason,
this document specifies two deployment scenarios for GRE-in-UDP
tunnels with GRE-in-UDP tunnel requirements for each of them: (1)
general Internet and (2) a traffic-managed controlled environment
(TMCE). Compared to the general Internet scenario, the TMCE scenario
has less restrictive technical requirements for the protocol but more
restrictive management and operation requirements for the network.

To provide security for traffic carried by a GRE-in-UDP tunnel, this
document also specifies Datagram Transport Layer Security (DTLS) for
GRE-in-UDP tunnels, which SHOULD be used when security is a concern.
GRE-in-UDP encapsulation usage requires no changes to the transit IP
network. ECMP hash functions in most existing IP routers may utilize
and benefit from the additional entropy enabled by GRE-in-UDP tunnels
without any change or upgrade to their ECMP implementation. The
encapsulation mechanism is applicable to a variety of IP networks
including Data Center Networks and Wide Area Networks, as well as
both IPv4 and IPv6 networks.
1.1. Terminology
The terms defined in [RFC768] and [RFC2784] are used in this
document. Below are additional terms used in this document.
Decapsulator: a component performing packet decapsulation at tunnel
egress.
ECMP: Equal-Cost Multipath.
Encapsulator: a component performing packet encapsulation at tunnel
egress.
Flow Entropy: The information to be derived from traffic or
applications and to be used by network devices in the ECMP process
[RFC6438].
Default GRE-in-UDP Tunnel: A GRE-in-UDP tunnel that can apply to the
general Internet.
TMCE: A traffic-managed controlled environment, i.e., an IP network
that is traffic-engineered and/or otherwise managed (e.g., via use of
traffic rate limiters) to avoid congestion, as defined in Section 2.
TMCE GRE-in-UDP Tunnel: A GRE-in-UDP tunnel that can only apply to a
traffic-managed controlled environment that is defined in Section 2.
Tunnel Egress: A tunnel endpoint that performs packet decapsulation.
Tunnel Ingress: A tunnel endpoint that performs packet encapsulation.
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

2. Applicability Statement
GRE-in-UDP encapsulation applies to IPv4 and IPv6 networks; in both
cases, encapsulated packets are treated as UDP datagrams. Therefore,
a GRE-in-UDP tunnel needs to meet the UDP usage requirements
specified in [RFC8085]. These requirements depend on both the
delivery network and the nature of the encapsulated traffic. For
example, the GRE-in-UDP tunnel protocol does not provide any
congestion control functionality beyond that of the encapsulated
traffic. Therefore, a GRE-in-UDP tunnel MUST be used only with
congestion-controlled traffic (e.g., IP unicast traffic) and/or
within a network that is traffic managed to avoid congestion.
[RFC8085] describes two applicability scenarios for UDP applications:
(1) general internet and (2) a controlled environment. The
controlled environment means a single administrative domain or
bilaterally agreed connection between domains. A network forming a
controlled environment can be managed/operated to meet certain
conditions, while the general Internet cannot be; thus, the
requirements for a tunnel protocol operating under a controlled
environment can be less restrictive than the requirements in the
general Internet.
For the purpose of this document, a traffic-managed controlled
environment (TMCE) is defined as an IP network that is traffic-
engineered and/or otherwise managed (e.g., via use of traffic rate
limiters) to avoid congestion.
This document specifies GRE-in-UDP tunnel usage in the general
Internet and GRE-in-UDP tunnel usage in a traffic-managed controlled
environment and uses "default GRE-in-UDP tunnel" and "TMCE GRE-in-UDP
tunnel" terms to refer to each usage.
NOTE: Although this document specifies two different sets of GRE-in-
UDP tunnel requirements based on tunnel usage, the tunnel
implementation itself has no ability to detect how and where it is
deployed. Therefore, it is the responsibility of the user or
operator who deploys a GRE-in-UDP tunnel to ensure that it meets the
appropriate requirements.
2.1. GRE-in-UDP Tunnel Requirements
This section states the requirements for a GRE-in-UDP tunnel.
Section 2.1.1 describes the requirements for a default GRE-in-UDP
tunnel that is suitable for the general Internet; Section 2.1.2
describes a set of relaxed requirements for a TMCE GRE-in-UDP tunnel
used in a traffic-managed controlled environment. Both Sections2.1.1 and 2.1.2 are applicable to an IPv4 or IPv6 delivery network.

2.1.1. Requirements for Default GRE-in-UDP Tunnel
The following is a summary of the default GRE-in-UDP tunnel
requirements:
1. A UDP checksum SHOULD be used when encapsulating in IPv4.
2. A UDP checksum MUST be used when encapsulating in IPv6.
3. GRE-in-UDP tunnel MUST NOT be deployed or configured to carry
traffic that is not congestion controlled. As stated in
[RFC8085], IP-based unicast traffic is generally assumed to be
congestion controlled, i.e., it is assumed that the transport
protocols generating IP-based traffic at the sender already
employ mechanisms that are sufficient to address congestion on
the path. A default GRE-in-UDP tunnel is not appropriate for
traffic that is not known to be congestion controlled (e.g., most
IP multicast traffic).
4. UDP source port values that are used as a source of flow entropy
SHOULD be chosen from the ephemeral port range (49152-65535)
[RFC8085].
5. The use of the UDP source port MUST be configurable so that a
single value can be set for all traffic within the tunnel (this
disables use of the UDP source port to provide flow entropy).
When a single value is set, a random port taken from the
ephemeral port range SHOULD be selected in order to minimize the
vulnerability to off-path attacks [RFC6056].
6. For IPv6 delivery networks, the flow entropy SHOULD also be
placed in the flow label field for ECMP per [RFC6438].
7. At the tunnel ingress, any fragmentation of the incoming packet
(e.g., because the tunnel has a Maximum Transmission Unit (MTU)
that is smaller than the packet) SHOULD be performed before
encapsulation. In addition, the tunnel ingress MUST apply the
UDP checksum to all encapsulated fragments so that the tunnel
egress can validate reassembly of the fragments; it MUST set the
same Differentiated Services Code Point (DSCP) value as in the
Differentiated Services (DS) field of the payload packet in all
fragments [RFC2474]. To avoid unwanted forwarding over multiple
paths, the same source UDP port value SHOULD be set in all packet
fragments.

2.1.2. Requirements for TMCE GRE-in-UDP Tunnel
The section contains the TMCE GRE-in-UDP tunnel requirements. It
lists the changed requirements, compared with a Default GRE-in-UDP
tunnel, for a TMCE GRE-in-UDP tunnel, which corresponds to
requirements 1-3 listed in Section 2.1.1.
1. A UDP checksum SHOULD be used when encapsulating in IPv4. A
tunnel endpoint sending GRE-in-UDP MAY disable the UDP checksum,
since GRE has been designed to work without a UDP checksum
[RFC2784]. However, a checksum also offers protection from
misdelivery to another port.
2. Use of the UDP checksum MUST be the default when encapsulating in
IPv6. This default MAY be overridden via configuration of UDP
zero-checksum mode. All usage of UDP zero-checksum mode with
IPv6 is subject to the additional requirements specified in
Section 6.2.
3. A GRE-in-UDP tunnel MAY encapsulate traffic that is not
congestion controlled.
Requirements 4-7 listed in Section 2.1.1 also apply to a TMCE GRE-in-
UDP tunnel.

3.1. IP Header
An encapsulator MUST encode its own IP address as the source IP
address and the decapsulator's IP address as the destination IP
address. A sufficiently large value is needed in the IPv4 TTL field
or IPv6 Hop Count field to allow delivery of the encapsulated packet
to the peer of the encapsulation.
3.2. UDP Header
3.2.1. Source Port
GRE-in-UDP permits the UDP source port value to be used to encode an
entropy value. The UDP source port contains a 16-bit entropy value
that is generated by the encapsulator to identify a flow for the
encapsulated packet. The port value SHOULD be within the ephemeral
port range, i.e., 49152 to 65535, where the high-order two bits of
the port are set to one. This provides fourteen bits of entropy for
the inner flow identifier. In the case that an encapsulator is
unable to derive flow entropy from the payload header or the entropy
usage has to be disabled to meet operational requirements (see
Section 7), to avoid reordering with a packet flow, the encapsulator
SHOULD use the same UDP source port value for all packets assigned to
a flow, e.g., the result of an algorithm that performs a hash of the
tunnel ingress and egress IP address.
The source port value for a flow set by an encapsulator MAY change
over the lifetime of the encapsulated flow. For instance, an
encapsulator may change the assignment for Denial-of-Service (DoS)
mitigation or as a means to effect routing through the ECMP network.
An encapsulator SHOULD NOT change the source port selected for a flow
more than once every thirty seconds.
An IPv6 GRE-in-UDP tunnel endpoint SHOULD copy a flow entropy value
in the IPv6 flow label field (requirement 6). This permits network
equipment to inspect this value and utilize it during forwarding,
e.g., to perform ECMP [RFC6438].
This document places requirements on the generation of the flow
entropy value [RFC8085] but does not specify the algorithm that an
implementation should use to derive this value.
3.2.2. Destination Port
The destination port of the UDP header is set to either GRE-in-UDP
(4754) or GRE-UDP-DTLS (4755); see Section 5.

3.2.3. Checksum
The UDP checksum is set and processed per [RFC768] and [RFC1122] for
IPv4 and per [RFC2460] for IPv6. Requirements for checksum handling
and use of zero UDP checksums are detailed in Section 6.
3.2.4. Length
The usage of this field is in accordance with the current UDP
specification in [RFC768]. This length will include the UDP header
(eight bytes), GRE header, and the GRE payload (encapsulated packet).
3.3. GRE Header
An encapsulator sets the protocol type (EtherType) of the packet
being encapsulated in the GRE Protocol Type field.
An encapsulator MAY set the GRE Key Present, Sequence Number Present,
and Checksum Present bits and associated fields in the GRE header as
defined by [RFC2784] and [RFC2890]. Usage of the reserved bits,
i.e., Reserved0, is specified in [RFC2784].
The GRE checksum MAY be enabled to protect the GRE header and
payload. When the UDP checksum is enabled, it protects the GRE
payload, resulting in the GRE checksum being mostly redundant.
Enabling both checksums may result in unnecessary processing. Since
the UDP checksum covers the pseudo-header and the packet payload,
including the GRE header and its payload, the UDP checksum SHOULD be
used in preference to the GRE checksum.
An implementation MAY use the GRE Key field to authenticate the
encapsulator. (See the Security Considerations section.) In this
model, a shared value is either configured or negotiated between an
encapsulator and decapsulator. When a decapsulator determines that a
presented key is not valid for the source, the packet MUST be
dropped.
Although the GRE-in-UDP encapsulation protocol uses both the UDP
header and GRE header, it is one tunnel encapsulation protocol. The
GRE and UDP headers MUST be applied and removed as a pair at the
encapsulation and decapsulation points. This specification does not
support UDP encapsulation of a GRE header where that GRE header is
applied or removed at a network node other than the UDP tunnel
ingress or egress.

4. Encapsulation Procedures
The procedures specified in this section apply to both a default GRE-
in-UDP tunnel and a TMCE GRE-in-UDP tunnel.
The GRE-in-UDP encapsulation allows encapsulated packets to be
forwarded through "GRE-in-UDP tunnels". The encapsulator MUST set
the UDP and GRE headers according to Section 3.
Intermediate routers, upon receiving these UDP encapsulated packets,
could load-balance these packets based on the hash of the five-tuple
of UDP packets.
Upon receiving these UDP encapsulated packets, the decapsulator
decapsulates them by removing the UDP and GRE headers and then
processes them accordingly.
GRE-in-UDP can encapsulate traffic with unicast, IPv4 broadcast, or
multicast (see requirement 3 in Section 2.1.1). However, a default
GRE-in-UDP tunnel MUST NOT be deployed or configured to carry traffic
that is not congestion-controlled (see requirement 3 in Section2.1.1). Entropy may be generated from the header of encapsulated
packets at an encapsulator. The mapping mechanism between the
encapsulated multicast traffic and the multicast capability in the IP
network is transparent and independent of the encapsulation and is
otherwise outside the scope of this document.
To provide entropy for ECMP, GRE-in-UDP does not rely on GRE keep-
alive. It is RECOMMENDED not to use GRE keep-alive in the GRE-in-UDP
tunnel. This aligns with middlebox traversal guidelines in
Section 3.5 of [RFC8085].
4.1. MTU and Fragmentation
Regarding packet fragmentation, an encapsulator/decapsulator SHOULD
perform fragmentation before the encapsulation. The size of
fragments SHOULD be less than or equal to the Path MTU (PMTU)
associated with the path between the GRE ingress and the GRE egress
tunnel endpoints minus the GRE and UDP overhead, assuming the egress
MTU for reassembled packets is larger than the PMTU. When applying
payload fragmentation, the UDP checksum MUST be used so that the
receiving endpoint can validate reassembly of the fragments; the same
source UDP port SHOULD be used for all packet fragments to ensure the
transit routers will forward the fragments on the same path.

If the operator of the transit network supporting the tunnel is able
to control the payload MTU size, the MTU SHOULD be configured to
avoid fragmentation, i.e., sufficient for the largest supported size
of packet, including all additional bytes introduced by the tunnel
overhead [RFC8085].
4.2. Differentiated Services and ECN Marking
To ensure that tunneled traffic receives the same treatment over the
IP network as traffic that is not tunneled, prior to the
encapsulation process, an encapsulator processes the tunneled IP
packet headers to retrieve appropriate parameters for the
encapsulating IP packet header such as Diffserv [RFC2983].
Encapsulation endpoints that support Explicit Congestion Notification
(ECN) must use the method described in [RFC6040] for ECN marking
propagation. The congestion control process is outside of the scope
of this document.
Additional information on IP header processing is provided in
Section 3.1.
5. Use of DTLS
Datagram Transport Layer Security (DTLS) [RFC6347] can be used for
application security and can preserve network- and transport-layer
protocol information. Specifically, if DTLS is used to secure the
GRE-in-UDP tunnel, the destination port of the UDP header MUST be set
to the IANA-assigned value (4755) indicating GRE-in-UDP with DTLS,
and that UDP port MUST NOT be used for other traffic. The UDP source
port field can still be used to add entropy, e.g., for load-sharing
purposes. DTLS applies to a default GRE-in-UDP tunnel and a TMCE
GRE-in-UDP tunnel.
Use of DTLS is limited to a single DTLS session for any specific
tunnel encapsulator/decapsulator pair (identified by source and
destination IP addresses). Both IP addresses MUST be unicast
addresses -- multicast traffic is not supported when DTLS is used. A
GRE-in-UDP tunnel decapsulator that supports DTLS is expected to be
able to establish DTLS sessions with multiple tunnel encapsulators,
and likewise a GRE-in-UDP tunnel encapsulator is expected to be able
to establish DTLS sessions with multiple decapsulators. Different
source and/or destination IP addresses will be involved; see
Section 6.2 for discussion of one situation where use of different
source IP addresses is important.

When DTLS is used for a GRE-in-UDP tunnel, if a packet is received
from the tunnel and that packet is not protected by the DTLS session
or part of DTLS negotiation (e.g., a DTLS handshake message
[RFC6347]), the tunnel receiver MUST discard that packet and SHOULD
log that discard event and information about the discarded packet.
DTLS SHOULD be used for a GRE-in-UDP tunnel to meet security
requirements of the original traffic that is delivered by a GRE-in-
UDP tunnel. There are cases where no additional security is
required, e.g., the traffic to be encapsulated is already encrypted
or the tunnel is deployed within an operationally secured network.
Use of DTLS for a GRE-in-UDP tunnel requires both tunnel endpoints to
configure use of DTLS.
6. UDP Checksum Handling
6.1. UDP Checksum with IPv4
For UDP in IPv4, when a non-zero UDP checksum is used, the UDP
checksum MUST be processed as specified in [RFC768] and [RFC1122] for
both transmit and receive. The IPv4 header includes a checksum that
protects against misdelivery of the packet due to corruption of IP
addresses. The UDP checksum potentially provides protection against
corruption of the UDP header, GRE header, and GRE payload. Disabling
the use of checksums is a deployment consideration that should take
into account the risk and effects of packet corruption.
When a decapsulator receives a packet, the UDP checksum field MUST be
processed. If the UDP checksum is non-zero, the decapsulator MUST
verify the checksum before accepting the packet. By default, a
decapsulator SHOULD accept UDP packets with a zero checksum. A node
MAY be configured to disallow zero checksums per [RFC1122]; this may
be done selectively, for instance, disallowing zero checksums from
certain hosts that are known to be sending over paths subject to
packet corruption. If verification of a non-zero checksum fails, a
decapsulator lacks the capability to verify a non-zero checksum, or a
packet with a zero checksum was received and the decapsulator is
configured to disallow, the packet MUST be dropped and an event MAY
be logged.
6.2. UDP Checksum with IPv6
For UDP in IPv6, the UDP checksum MUST be processed as specified in
[RFC768] and [RFC2460] for both transmit and receive.
When UDP is used over IPv6, the UDP checksum is relied upon to
protect both the IPv6 and UDP headers from corruption. As such, a
default GRE-in-UDP tunnel MUST perform UDP checksum; a TMCE GRE-in-

UDP tunnel MAY be configured with UDP zero-checksum mode if the
traffic-managed controlled environment or a set of closely
cooperating traffic-managed controlled environments (such as by
network operators who have agreed to work together in order to
jointly provide specific services) meet at least one of the following
conditions:
a. It is known (perhaps through knowledge of equipment types and
lower-layer checks) that packet corruption is exceptionally
unlikely and where the operator is willing to take the risk of
undetected packet corruption.
b. It is judged through observational measurements (perhaps of
historic or current traffic flows that use a non-zero checksum)
that the level of packet corruption is tolerably low and where
the operator is willing to take the risk of undetected packet
corruption.
c. Carrying applications that are tolerant of misdelivered or
corrupted packets (perhaps through higher-layer checksum,
validation, and retransmission or transmission redundancy) where
the operator is willing to rely on the applications using the
tunnel to survive any corrupt packets.
The following requirements apply to a TMCE GRE-in-UDP tunnel that
uses UDP zero-checksum mode:
a. Use of the UDP checksum with IPv6 MUST be the default
configuration of all GRE-in-UDP tunnels.
b. The GRE-in-UDP tunnel implementation MUST comply with all
requirements specified in Section 4 of [RFC6936] and with
requirement 1 specified in Section 5 of [RFC6936].
c. The tunnel decapsulator SHOULD only allow the use of UDP zero-
checksum mode for IPv6 on a single received UDP Destination Port,
regardless of the encapsulator. The motivation for this
requirement is possible corruption of the UDP Destination Port,
which may cause packet delivery to the wrong UDP port. If that
other UDP port requires the UDP checksum, the misdelivered packet
will be discarded.
d. It is RECOMMENDED that the UDP zero-checksum mode for IPv6 is
only enabled for certain selected source addresses. The tunnel
decapsulator MUST check that the source and destination IPv6
addresses are valid for the GRE-in-UDP tunnel on which the packet
was received if that tunnel uses UDP zero-checksum mode and
discard any packet for which this check fails.

e. The tunnel encapsulator SHOULD use different IPv6 addresses for
each GRE-in-UDP tunnel that uses UDP zero-checksum mode,
regardless of the decapsulator, in order to strengthen the
decapsulator's check of the IPv6 source address (i.e., the same
IPv6 source address SHOULD NOT be used with more than one IPv6
destination address, independent of whether that destination
address is a unicast or multicast address). When this is not
possible, it is RECOMMENDED to use each source IPv6 address for
as few GRE-in-UDP tunnels that use UDP zero-checksum mode as is
feasible.
f. When any middlebox exists on the path of a GRE-in-UDP tunnel, it
is RECOMMENDED to use the default mode, i.e., use UDP checksum,
to reduce the chance that the encapsulated packets will be
dropped.
g. Any middlebox that allows the UDP zero-checksum mode for IPv6
MUST comply with requirements 1 and 8-10 in Section 5 of
[RFC6936].
h. Measures SHOULD be taken to prevent IPv6 traffic with zero UDP
checksums from "escaping" to the general Internet; see Section 8
for examples of such measures.
i. IPv6 traffic with zero UDP checksums MUST be actively monitored
for errors by the network operator. For example, the operator
may monitor Ethernet-layer packet error rates.
j. If a packet with a non-zero checksum is received, the checksum
MUST be verified before accepting the packet. This is regardless
of whether the tunnel encapsulator and decapsulator have been
configured with UDP zero-checksum mode.
The above requirements do not change either the requirements
specified in [RFC2460] as modified by [RFC6935] or the requirements
specified in [RFC6936].
The requirement to check the source IPv6 address in addition to the
destination IPv6 address and the strong recommendation against reuse
of source IPv6 addresses among GRE-in-UDP tunnels collectively
provide some mitigation for the absence of UDP checksum coverage of
the IPv6 header. A traffic-managed controlled environment that
satisfies at least one of three conditions listed at the beginning of
this section provides additional assurance.
A GRE-in-UDP tunnel is suitable for transmission over lower layers in
the traffic-managed controlled environments that are allowed by the
exceptions stated above, and the rate of corruption of the inner IP

packet on such networks is not expected to increase by comparison to
GRE traffic that is not encapsulated in UDP. For these reasons, GRE-
in-UDP does not provide an additional integrity check except when GRE
checksum is used when UDP zero-checksum mode is used with IPv6, and
this design is in accordance with requirements 2, 3, and 5 specified
in Section 5 of [RFC6936].
Generic Router Encapsulation (GRE) does not accumulate incorrect
transport-layer state as a consequence of GRE header corruption. A
corrupt GRE packet may result in either packet discard or packet
forwarding without accumulation of GRE state. Active monitoring of
GRE-in-UDP traffic for errors is REQUIRED, as the occurrence of
errors will result in some accumulation of error information outside
the protocol for operational and management purposes. This design is
in accordance with requirement 4 specified in Section 5 of [RFC6936].
The remaining requirements specified in Section 5 of [RFC6936] are
not applicable to GRE-in-UDP. Requirements 6 and 7 do not apply
because GRE does not include a control feedback mechanism.
Requirements 8-10 are middlebox requirements that do not apply to
GRE-in-UDP tunnel endpoints. (See Section 7.1 for further middlebox
discussion.)
It is worth mentioning that the use of a zero UDP checksum should
present the equivalent risk of undetected packet corruption when
sending a similar packet using GRE-in-IPv6 without UDP [RFC7676] and
without GRE checksums.
In summary, a TMCE GRE-in-UDP tunnel is allowed to use UDP zero-
checksum mode for IPv6 when the conditions and requirements stated
above are met. Otherwise, the UDP checksum needs to be used for IPv6
as specified in [RFC768] and [RFC2460]. Use of GRE checksum is
RECOMMENDED when the UDP checksum is not used.
7. Middlebox Considerations
Many middleboxes read or update UDP port information of the packets
that they forward. Network Address Port Translator (NAPT) is the
most commonly deployed Network Address Translation (NAT) device
[RFC4787]. A NAPT device establishes a NAT session to translate the
{private IP address, private source port number} tuple to a {public
IP address, public source port number} tuple, and vice versa, for the
duration of the UDP session. This provides a UDP application with
the "NAT pass-through" function. NAPT allows multiple internal hosts
to share a single public IP address. The port number, i.e., the UDP
Source Port number, is used as the demultiplexer of the multiple

internal hosts. However, the above NAPT behaviors conflict with the
behavior of a GRE-in-UDP tunnel that is configured to use the UDP
source port value to provide entropy.
A GRE-in-UDP tunnel is unidirectional; the tunnel traffic is not
expected to be returned back to the UDP source port values used to
generate entropy. However, some middleboxes (e.g., firewalls) assume
that bidirectional traffic uses a common pair of UDP ports. This
assumption also conflicts with the use of the UDP source port field
as entropy.
Hence, use of the UDP source port for entropy may impact middleboxes'
behavior. If a GRE-in-UDP tunnel is expected to be used on a path
with a middlebox, the tunnel can be configured either to disable use
of the UDP source port for entropy or to enable middleboxes to pass
packets with UDP source port entropy.
7.1. Middlebox Considerations for Zero Checksums
IPv6 datagrams with a zero UDP checksum will not be passed by any
middlebox that updates the UDP checksum field or simply validates the
checksum based on [RFC2460], such as firewalls. Changing this
behavior would require such middleboxes to be updated to correctly
handle datagrams with zero UDP checksums. The GRE-in-UDP
encapsulation does not provide a mechanism to safely fall back to
using a checksum when a path change occurs that redirects a tunnel
over a path that includes a middlebox that discards IPv6 datagrams
with a zero UDP checksum. In this case, the GRE-in-UDP tunnel will
be black-holed by that middlebox.
As such, when any middlebox exists on the path of a GRE-in-UDP
tunnel, use of the UDP checksum is RECOMMENDED to increase the
probability of successful transmission of GRE-in-UDP packets.
Recommended changes to allow firewalls and other middleboxes to
support use of an IPv6 zero UDP checksum are described in Section 5
of [RFC6936].
8. Congestion Considerations
Section 3.1.9 of [RFC8085] discusses the congestion considerations
for design and use of UDP tunnels; this is important because other
flows could share the path with one or more UDP tunnels,
necessitating congestion control [RFC2914] to avoid destructive
interference.
Congestion has potential impacts both on the rest of the network
containing a UDP tunnel and on the traffic flows using the UDP
tunnels. These impacts depend upon what sort of traffic is carried

over the tunnel, as well as the path of the tunnel. The GRE-in-UDP
tunnel protocol does not provide any congestion control and GRE-in-
UDP packets are regular UDP packets. Therefore, a GRE-in-UDP tunnel
MUST NOT be deployed to carry non-congestion-controlled traffic over
the Internet [RFC8085].
Within a TMCE network, GRE-in-UDP tunnels are appropriate for
carrying traffic that is not known to be congestion controlled. For
example, a GRE-in-UDP tunnel may be used to carry Multiprotocol Label
Switching (MPLS) traffic such as pseudowires or VPNs where specific
bandwidth guarantees are provided to each pseudowire or VPN. In such
cases, operators of TMCE networks avoid congestion by careful
provisioning of their networks, rate-limiting of user data traffic,
and traffic engineering according to path capacity.
When a GRE-in-UDP tunnel carries traffic that is not known to be
congestion controlled in a TMCE network, the tunnel MUST be deployed
entirely within that network, and measures SHOULD be taken to prevent
the GRE-in-UDP traffic from "escaping" the network to the general
Internet. Examples of such measures are:
o physical or logical isolation of the links carrying GRE-in-UDP
from the general Internet,
o deployment of packet filters that block the UDP ports assigned for
GRE-in-UDP, and
o imposition of restrictions on GRE-in-UDP traffic by software tools
used to set up GRE-in-UDP tunnels between specific end systems (as
might be used within a single data center) or by tunnel ingress
nodes for tunnels that don't terminate at end systems.
9. Backward Compatibility
In general, tunnel ingress routers have to be upgraded in order to
support the encapsulations described in this document.
No change is required at transit routers to support forwarding of the
encapsulation described in this document.
If a tunnel endpoint (a host or router) that is intended for use as a
decapsulator does not support or enable the GRE-in-UDP encapsulation
described in this document, that endpoint will not listen on the
destination port assigned to the GRE-encapsulation (4754 and 4755).
In these cases, the endpoint will perform normal UDP processing and
respond to an encapsulator with an ICMP message indicating "port

unreachable" according to [RFC792]. Upon receiving this ICMP
message, the node MUST NOT continue to use GRE-in-UDP encapsulation
toward this peer without management intervention.
10. IANA Considerations
IANA has allocated the following UDP destination port number for the
indication of GRE:
Service Name: GRE-in-UDP
Transport Protocol(s): UDP
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: GRE-in-UDP Encapsulation
Reference: RFC 8086
Port Number: 4754
Service Code: N/A
Known Unauthorized Uses: N/A
Assignment Notes: N/A
IANA has allocated the following UDP destination port number for the
indication of GRE with DTLS:
Service Name: GRE-UDP-DTLS
Transport Protocol(s): UDP
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: GRE-in-UDP Encapsulation with DTLS
Reference: RFC 8086
Port Number: 4755
Service Code: N/A
Known Unauthorized Uses: N/A
Assignment Notes: N/A
11. Security Considerations
GRE-in-UDP encapsulation does not affect security for the payload
protocol. The security considerations for GRE apply to GRE-in-UDP;
see [RFC2784].
To secure traffic carried by a GRE-in-UDP tunnel, DTLS SHOULD be used
as specified in Section 5.
In the case that UDP source port for entropy usage is disabled, a
random port taken from the ephemeral port range SHOULD be selected in
order to minimize the vulnerability to off-path attacks [RFC6056].
The random port may also be periodically changed to mitigate certain
DoS attacks as mentioned in Section 3.2.1.

Using one standardized value as the UDP destination port to indicate
an encapsulation may increase the vulnerability to off-path attacks.
To overcome this, an alternate port may be agreed upon to use between
an encapsulator and decapsulator [RFC6056]. How the encapsulator
endpoints communicate the value is outside the scope of this
document.
This document does not require that a decapsulator validate the IP
source address of the tunneled packets (with the exception that the
IPv6 source address MUST be validated when UDP zero-checksum mode is
used with IPv6), but it should be understood that failure to do so
presupposes that there is effective destination-based filtering (or a
combination of source-based and destination-based filtering) at the
boundaries.
Corruption of GRE headers can cause security concerns for
applications that rely on the GRE Key field for traffic separation or
segregation. When the GRE Key field is used for this purpose, such
as an application of a Network Virtualization Using Generic Routing
Encapsulation (NVGRE) [RFC7637], GRE header corruption is a concern.
In such situations, at least one of the UDP and GRE checksums MUST be
used for both IPv4 and IPv6 GRE-in-UDP tunnels.
12. References
12.1. Normative References
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<http://www.rfc-editor.org/info/rfc768>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<http://www.rfc-editor.org/info/rfc2474>.

Acknowledgements
The authors would like to thank Vivek Kumar, Ron Bonica, Joe Touch,
Ruediger Geib, Lars Eggert, Lloyd Wood, Bob Briscoe, Rick Casarez,
Jouni Korhonen, Kathleen Moriarty, Ben Campbell, and many others for
their reviews and valuable input on this document.
Thanks to Donald Eastlake, Eliot Lear, Martin Stiemerling, and
Spencer Dawkins for their detailed reviews and valuable suggestions
during WG Last Call and the IESG process.
Thanks to the design team led by David Black (members: Ross Callon,
Gorry Fairhurst, Xiaohu Xu, and Lucy Yong) for efficiently working
out the descriptions for the congestion considerations and IPv6 UDP
zero checksum.
Thanks to David Black and Gorry Fairhurst for their great help in
document content and editing.
Contributors
The following people all contributed significantly to this document
and are listed in alphabetical order:
David Black
EMC Corporation
176 South Street
Hopkinton, MA 01748
United States of America
Email: david.black@emc.com
Ross Callon
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
United States of America
Email: rcallon@juniper.net
John E. Drake
Juniper Networks
Email: jdrake@juniper.net